The best way to learn (especially if you're preparing for the SCJP exam) is to write lots of test code. The answer to this question can be easily demonstrated with a few lines of code.
But before preparing for the SCJP exam, you need to get the fundamentals down. I think questions like this would be more appropriate in the Java In General (Beginners) forum. [ September 07, 2007: Message edited by: marc weber ]
"We're kind of on the level of crossword puzzle writers... And no one ever goes to them and gives them an award." ~Joe Strummer sscce.org